1. Field of Art
This description generally relates to photovoltaic solar cells, and particularly to metal layers used as electrical interconnections in solar cells as well as cell-to-cell interconnections in solar modules.
2. Description of the Related Art
There are a number of existing types of photovoltaic solar cells. One type of solar cell design is an interdigitated back contact (IBC) solar cell (or back contact solar cell in short). A back contact solar cell may be advantageous over a front contact solar cell in that both the base and emitter contacts of the solar cell are located on the back side of a semiconductor light absorbing layer (opposite the sunnyside of the solar cell). As a result, light incident on the front side of the light absorbing layer is not obstructed by a front side electrical metallization grid. This has the beneficial effect of increasing the amount of sunlight that may be received by the solar cell per unit area of solar cell surface area (due to elimination of optical shading losses). To accommodate both base and emitter contacts on the back side of the light absorbing semiconductor layer, the corresponding base and emitter metallization fingers are commonly interdigitated relative to each other, hence the name IBC solar cell.
Metallized base and emitter contacts on the back surface of the light absorbing semiconductor layer are typically low resistance electrically conductive (e.g., metal) contacts designed to carry charges away from the light absorbing layers as photogenerated electrons and holes selectively arriving at the electrically conductive contacts from the light absorbing semiconductor layer. To reduce recombination at the junction where the metallized contacts meet the light absorbing semiconductor layer, generally small metallized contacts (with relatively small area fraction) in physical contact with the light absorbing semiconductor layer may be advantageous. However, thin metallization lines also may have relatively high electrical ohmic resistance, particularly given the distance which the electrical current must be carried away from the individual solar cells (for base and emitter) to an external connection (e.g., solar cell busbar). To address this issue, IBC solar cells may use a dual-layer (or two-level) metallization structure, wherein a first-level metallization, also known as contact metallization (M1), is designed to reduce contact recombination and to extract the solar cell power (e.g., typically a finer pitch and thinner metallization relative to M2), and a charge carrying metallization (M2) is designed for low resistance (e.g., thicker with higher sheet conductance relative to M1) and long-range power transport out of the solar cell (and interconnections between the solar cells in a solar module). The M1 and M2 layers may be separated by an electrically insulating layer, and connected in designated regions by via holes drilled (e.g., by laser) in the insulating layer.
Often, an M2 layer may be formed via some form of physical vapor deposition (PVD). While PVD has the advantage of allowing for an in-situ clean step or surface etch step prior to M2 deposition, thus allowing for the removal of oxide from the M1 layer and allowing for a strong physical and electrical connection between the M1 and M2 layers, it may be expensive to perform. And while PVD is perfectly suitable for use in depositing relatively thin layers such as the M1 layer, PVD costs may scale up when depositing relatively thick layers such as the M2 layer. Generally, the thicker the M2 layer, the lower the resistance and thus the better the performance of the M2 layer, however the greater the cost of the solar cell as a whole.
The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein.